Wound golf balls are the preferred ball of more advanced players due to their spin and feel characteristics. Wound balls typically have either a solid rubber or fluid-filled center around which a wound layer is formed, which results in a wound core. The wound layer is formed of thread that is stretched and wrapped about the center. The wound core is then covered with a durable cover material, such as a SURLYN.RTM. or similar material, or a softer "performance" cover, such as Balata or polyurethane.
Wound balls are generally softer and provide more spin than solid balls, which enables a skilled golfer to have more control over the ball's flight and final position. Particularly, with approach shots into the green, the high spin rate of soft covered, wound balls enables the golfer to stop the ball very near its landing position. In addition, wound balls exhibit lower compression than two piece balls, however, their higher spin rate means wound balls generally display shorter distance than hard covered solid balls. However, the advantages of wound constructions over solid ones are more related to targeting or accuracy than distance.
The United States Golf Association (USGA), the organization that sets the rules of golf in the United States, has instituted a rule that prohibits the competitive use in any USGA sanctioned event of a golf ball that can achieve an initial velocity of 76.2 meters per second (m/s), or 250 ft/s, when struck by a driver with a velocity of 39.6 m/s, i.e., 130 ft/s (referred to hereinafter as "the USGA test"). However, an allowed tolerance of 2 percent permits manufacturers to produce golf balls that achieve an initial velocity of 77.7 m/s (255 ft/s).
Players generally seek a golf ball that delivers maximum distance, which requires a high initial velocity upon impact. Therefore, in an effort to meet the demands of the marketplace, manufacturers strive to produce golf balls with initial velocities in the USGA test that approximate the USGA maximum of 77.7 m/s or 255 ft/s as closely as possible. Manufacturers try to provide these balls with a range of different properties and characteristics, such as spin, compression, and "feel."
To meet the needs of golfers with various levels of skill, golf ball manufacturers are also concerned with varying the level of the compression of the ball, which is a measurement of the deformation of a golf ball under a fixed load. A ball with a higher compression feels harder than a ball of lower compression. Wound golf balls generally have a lower compression which is preferred by better players. Whether wound or solid, all golf balls become more resilient (i.e., have higher initial velocities) as compression increases. Manufacturers of both wound and solid construction golf balls must balance the requirement of higher initial velocity from higher compression with the desire for a softer feel from lower compression.
To make wound golf balls, manufacturers use automated winding machines to stretch the threads to various degrees of elongation during the winding process without subjecting the threads to unnecessary incidents of breakage. As the elongation and the winding tension increases, the compression and initial velocity of the ball increases. Thus, a more lively wound ball is produced, which is desirable.
Referring to FIG. 1, a conventional single-ply golf ball thread 10 is shown. In general, the single-ply golf ball thread 10 is formed by mixing synthetic cis,polyisoprene rubbers, natural rubber and a curing system together, calendering this mixture into a sheet, curing the sheet, and slitting the sheet into threads.
Referring to FIG. 2, a conventional two-ply golf ball thread 20 is shown. In the case of the two-ply golf ball thread, the mixture and calendering steps are the same as in the single-ply thread. However, after the sheets are thus formed, they are calendered together, cured to bond the plies or sheets together, and slit into threads. Each ply of the thread 20 has a thickness t.sub.1 and t.sub.2. Generally, these thicknesses are substantially the same, and each ply also has the same physical properties.
For golf balls the thread is typically formed by a calender method rather than an extrusion method, the calendered thread has a rectangular cross-section, while extruded thread generally has a circular cross-section. Extruded thread has not been used in golf ball applications, because it has not exhibited the physical properties necessary for proper performance of golf balls. An example of an extruded thread that is not used in golf balls is disclosed in U.S. Pat. No. 5,679,196 issued to Wilhelm et al. This patent discloses a thread formed of a mixture that has more than 50% natural rubber.
There are some drawbacks to the conventional threads used in golf balls. The single-ply or each ply of the two-ply thread occasionally contains weak points. As a result, manufacturers of wound balls do not wind using the maximum tension or stretch the thread to the maximum elongation, because to do so would cause an excessive amount of breakage during winding.
In the case of the two-ply thread, when one ply breaks typically the other ply also breaks, since the plies have the same physical properties. When a thread breaks during manufacturing, if the winding machine does not lose control of the free end of the thread, the machine needs to be restarted. However, if the winding machine loses control of the free end of the thread, an operator must manually re-thread the machine and restart the operation. Both of these situations decrease production, and thus are undesirable.
The thread can also break during play due to impact of a club with the ball. These breaks can result in various consequences. The cover material is disposed between the threads adjacent the cover. When the threads adjacent the cover break, the cover material tends to hold these threads in the proper position. However, if enough threads break near the cover, a lump will be created on the outside surface of the ball, which makes the ball unplayable.
More severe problems can occur, when the threads near the center break. In a wound ball with a solid rubber center, the resilient rubber of the center is relatively soft compared to the hardness of the highly stretched threads. After a thread adjacent the center breaks, the thread can contract and cause a loss of compression and initial velocity. This results in a short shot, which is undesirable.
In a wound ball with a fluid-filled center, after a thread adjacent the center breaks, if the thread unravels and contracts due to relaxation of the tension, the thread cuts through the envelope that contains the fluid. This destroys the structural integrity of the ball and makes it unplayable. If this type of failure happens during a shot, it can result in a short shot. It can also result in the ball deviating from its line of flight as it leaves the club, so that the ball can end up off of the fairway. Both of these consequences are undesirable. Similar problems occur, when a single-ply thread breaks.
Therefore, golf ball manufacturers are continually searching for new ways in which to provide wound golf balls that deliver the maximum performance for golfers while decreasing the occurrence of thread breaks both during manufacturing and during play. It would be advantageous to provide a wound golf ball with a higher compression, higher initial velocity, improved durability, and improved manufacturing processibility. The present invention provides such a wound golf ball.